Viessmann Vitodens 100W - WB1B, F2 temperature limiter fault

Over the last couple of months the boiler has stopped 3 or 4 times with an F2 error code.
Usually it is first thing in the morning, or in the really cold spell, when the frost stat kicked in during the early hours. I have begun to recognise the symptoms as radiators cold, but pump running continuously, and pipes into the pump also cold.

According to the service booklet, F2 is a "temperature limiter" error, and the things to check are:
•system level full
•circulation pump working
•temperature limiter and wiring.

I have checked the system level, and I know the pump is running, but the pump is very likely the original pump and getting on for 30 years old. It's a Grundfos something, but I am wondering about its efficiency, since the furthest radiator does take a while to get hot.

On reset, the boiler display changes from "F2" flashing, to "141", followed by "115degC", then normal operation resumes.